Styrene is a basic building block for the manufacture of a broad range of materials. It is used to make polystyrene, acrylonitrile-butadiene-styrene, polyester resins, synthetic rubber, and a host of other products.
Production of styrene by dehydrogenation of ethylbenzene (EB) is commonly conducted by mixing ethylbenzene with steam and passing the mixture through a dehydrogenation catalyst-packed bed at elevated temperature (600-650° C. at the inlet). Steam is used as the diluent gas in the dehydrogenation reaction system to supply heat needed for the endothermic reaction of ethylbenzene to styrene (SM).
The steam used as the diluent has several other functions, e.g., it supplies the heat necessary for dehydrogenation, reduces the partial pressure of the reactants, and removes carbon on the catalyst as carbon monoxide, which is subsequently converted to carbon dioxide via the water gas shift reaction. It is difficult to recover the heat not used in the reaction from the steam and a huge amount of heat is left unrecovered when steam is used in large volume. The reduction of the amount of steam used in the dehydrogenation of ethylbenzene is a greatly desired benefit to process economy and several attempts have been made to achieve this end.
Current methods for the production of styrene via dehydrogenation of ethylbenzene utilize not less than 0.8 kg of steam per kg of ethylbenzene to bring the reactor feed to the required temperature, and to reheat the effluent between reactors, which is needed because the dehydrogenation of ethylbenzene is highly endothermic. This minimum amount of steam is necessary to keep steam temperature below 899° C., which is the maximum allowable temperature for the standard materials, such as Alloy 800H, used for fabrication of high temperature process equipment and transfer lines. Reducing steam/ethylbenzene ratio to less than 0.8 kg/kg molar would require use of very expensive alloys, which are unproven in the styrene service.
Alternate methods for avoiding high steam temperature in processes for producing styrene via dehydrogenation of ethylbenzene are known in the art. For example, U.S. Pat. No. 8,084,660 discloses a method for increasing the efficiency and/or expanding the capacity of a new or existing dehydrogenation section of a styrene plant by adding a direct heating unit to the dehydrogenation section having a reheater. The direct heating unit is positioned before or after the reactor, and the direct heating unit and reheater are operated in a parallel arrangement with respect to each other. The reactor effluent is diverted to both the direct heating unit and the reheater for heating. Operating the dehydrogenation section with an added direct heating unit provides energy savings, as compared to operating a dehydrogenation section with only a reheater.
U.S. Patent Application Publication No. 2010/0240940 discloses a method for the production of styrene by the catalytic dehydrogenation of ethylbenzene employing diluent steam at a steam to oil ratio that can be 1.0 or below. The method utilizes steam temperatures at the outlet of the steam superheater below those that would require the use of special and costly metallurgy in the high temperature process equipment. Moreover, this disclosure relies on the idea of increasing the flow of heating steam through the system without actually using more steam. This is accomplished by recirculating a portion of the heating steam by means of a compressor or a steam ejector. The compressor option seems to be less favored by the inventors, presumably due to the high cost and questionable reliability of rotary equipment operating at temperatures in excess of 600° C. The steam ejector option requires that the make-up heating steam be supplied at high pressure, which is not feasible and/or economic in a scenario where ethylbenzene and styrene production facilities are integrated; given that the ethylbenzene process produces large amounts of low pressure steam, for which the styrene process provides an outlet. This is true for the vast majority of styrene produced via dehydrogenation of ethylbenzene today.
For economic reasons there remains a need in the industry for methods and systems which can produce styrene via dehydrogenation of ethylbenzene utilizing less than about 0.8 kg of steam per kg of ethylbenzene.